Digital Modes for Amateur Radio

Introduction

People can use amateur radio not just for voice communication, but also for digital communication. This happens all over the bands, from HF through UHF. On VHF and UHF, digital might be D-Star or packet, which are not discussed here. This page mainly discusses the keyboard-to-keyboard HF modes.

These modes are used for keyboard-to-keyboard communication. That is, you are having a QSO (conversation) with another amateur using keyboards, similar to the concept of an instant message conversation.

What You Need

To operate with HF digital, most people will use a PC with some sort of audio interface to their radio. Cables can be made to hook up your sound card to the radio. Many people instead go for a SignaLink USB or similar interface, which is about $100 but doesn't interfere with the regular use of your sound card, and probably provides a higher-quality interface as well.

For software: You will use a sound card interface software for this. I use fldigi because it is cross-platform (Linux, FreeBSD, Mac, and Windows), high quality, and free. Other options such as !MixW and !MultiPSK exist as well. The examples in this page refer to fldigi, because it's what I know, but the concepts should translate well to others.

Basics of Operating

There are many different digital modes. PSK-31 is probably the most common. You can often find PSK-31 at these frequencies:

3.580 USB

7.035 USB

7.050 USB

10.142 USB

14.070 USB

18.100 USB

21.070 USB

21.080 USB

24.920 USB

28.120 USB

Other popular modes include RTTY and MFSK-16. Even PSK can be used in faster modes; for instance, PSK-63 runs at 63 baud.

For now, we'll assume PSK-31. Please note: the most common PSK-31 mode is often called BPSK31 in software (QPSK is usually not what you want).

The Waterfall

Normally at the bottom of the display in your interface software, you'll see a waterfall. It looks like this:

A PSK-31 signal is much narrower -- instead of a bandwidth of 3000Hz, it only uses 31.25Hz. So, when you tune your rig to a given frequency in USB mode, you can receive dozens of PSK-31 signals at once. If you listen to the audio, they will sound like they are simply different pitches; lower frequency will be lower-pitched, higher frequency will be higher pitched. This is similar to the way that you can distinctly hear both a soprano and a baritone singing at the same time.

So, since tuning your rig doesn't select one individual PSK-31 signal, but rather a 3kHz-wide window of many, many PSK-31 signals, you need a way to select the PSK-31 signal you want in software. This is where the waterfall comes in. The fldigi waterfall represents, by default, the 3kHz bandwidth that your rig will theoretically receive. (In practice, filters and the like reduce that width somewhat, down to perhaps the middle 2kHz.)

A few times a second, a new row of pixels is added at the top of the waterfal. The pixel at any given location represents the strength of a received signal at that frequency -- brighter for brighter signals. So what you see is a recent history of transmissions across the 3kHz bandwidth. Ongoing transmissions will appear as vertical bars, and you can click on one to tune to it and decode it. In this example, an MFSK-16 transmission is being decoded; the red bars represent the boundary of the signal after the operator clicked on it.

Different modes look different on your waterfall. Here are some links to pages with screenshots of the appearnces of different modes:

One other feature of fldigi and many other programs is what fldigi calls the "PSK browser". It will simultaneously decode all PSK streams on your waterfall, and can optionally highlight ones that are calling CQ so you can easily find a station to talk to.

I'm not going to go into more detail about operating software here because there are plenty of good references available via Google or the online help for your program.

Operating Practices

There are a few things to keep in mind on digital:

At any given moment, only one station can transmit. This is similar to voice on HF, and there are similar challenges: needing to make sure each station knows when the other is done transmitting, and ready to listen to your transmission.

Some of the modes are fairly slow. PSK-31 is roughly 50 words per minute, which is slower than the typing speed of many touch typists.

The need to identify yourself still exists

PSK-31 is not an error-free mode. You can see corruption in the received text.

As such, there are some practices and terms that help people.

Digital Lingo

Much of the link is related to CW (Morse) conventions. There are plenty of lists, but I want to give you just a few that you'll need to know for PSK-31 operation.

Term

Meaning

CQ

General call to any station

CQ DX

Calling any station that is DX (long distance) from this one

CQ location

Calling any station in the given location

pse

Please

tnx

Thanks

BTU

Back To You

de

From

K

Ending my transmission

KN

Ending my transmission, go ahead station I'm QSOing with

SK

Silent Key (ending my transmission with this contact)

QRZ?

Who (else) is calling/would like to talk to me?

U R

You are

A sample QSO

Here's a sample conversation. I'll call the stations BB0BB and CC0CC just for example, and label the transmissions appropriately. First, BB0BB is calling CQ.

Like a SSB CQ, you want this to be long enough that people find it. Also, the callsign is important to get right, so it specifically is repeated multiple times in case it gets corrupted due to noise or QSB.

Some software have macros, so you can hit a certain key to transmit often-needed information, such as the BTU line, the information about names, rigs, etc. Both people may have used macros in this QSO. So let's say that the QSO has gone on for awhile and they're ready to say goodbye. Here's how that may happen:

BB0BB transmits:
CC0CC de BB0BB
Well OM, time for me to go. 73 73 to you and have a good weekend.
CC0CC de BB0BB SK SK SK